A conventional oil pump includes: an inner rotor having n (n is a natural number) external teeth; an outer rotor having n+1 internal teeth that are engageable with the external teeth; and a casing having an intake port for drawing in a fluid and a discharge port for discharging the same. Particularly, the external teeth and the internal teeth engage with one another as the inner rotor rotates, thereby allowing the outer rotor to rotate such that a fluid can be drawn in and discharged as volumes of a plurality of cells formed between the two rotors change.
The cells are individually established as the external teeth of the inner rotor and the internal teeth of the outer rotor individually come into contact with one another on a forward side and a backward side of a rotational direction. Further, each cell has both of its side surfaces surrounded by the casing. Thus, the cells are configured as individual fluid transferring chambers. Particularly, each cell draws in a fluid as the volume thereof enlarges when moving along the intake port, after the volume of the corresponding cell has reached its minimum level during the process of engaging the external teeth and the internal teeth with one another. In contrast, the cell discharges the fluid as the volume thereof decreases when moving along the discharge port, after the volume of the corresponding cell has reached its maximum level during the aforementioned process.
Since an oil pump configured as above is small and has a simple structure, it can be widely used as, for example, a lubricating oil pump and an automatic transmission oil pump that are installed in automobiles. When used in an automobile, the oil pump is driven by, for example, allowing the inner rotor to be directly coupled to a crankshaft of an engine such that the oil pump can be driven as the engine rotates; or the oil pump may also be driven by, for example, allowing the inner rotor to be coupled to an electric motor.
As for the aforementioned oil pump, for the purpose of reducing the noise of the pump and improving a mechanical efficiency, tip clearances of an appropriate size are provided between the tooth tips of the inner rotor and the tooth tips of the outer rotor at where the inner rotor and the outer rotor, while being coupled to each other, have been rotated by 180° from an engagement point.
Here, the conditions required for determining the tooth shapes of an inner rotor ri and an outer rotor ro are as follows. That is, as for the inner rotor ri, rolling distances of a first outer rolling circle Di′ (diameter ΦDi′) and a first inner rolling circle di′ (diameter Φdi′) should add up to one cycle. That is, the rolling distances of the first outer rolling circle Di′ and the first inner rolling circle di′ should altogether be equal to the circumference of a base circle bi′ (diameter Φbi′) of the inner rotor ri, and henceΦ bi′=n·(Φ Di′+Φ di′)
Likewise, as for the outer rotor ro, rolling distances of a second outer rolling circle Do′ (diameter ΦDo′) and a second inner rolling circle do′ (diameter Φdo′) should altogether be equal to the circumference of a base circle bo′ (diameter Φbo′) of the outer rotor ro, and hence.Φ bo′=(n+1)·(Φ Do′+Φ do′)Next, since the inner rotor ri and the outer rotor ro are to be engaged with each other, the expression    Φ Di′+Φ di′=Φ Do′+Φ do′=2 e′ holds, provided that an eccentricity amount of the two rotors ri and ro is e′. Based on the aforementioned expressions, the expression    n·Φ bo′=(n+1)·Φ bi′ holds. The tooth shapes of the inner rotor ri and the outer rotor ro are configured to satisfy these requirements. Here, by satisfying the expressionsΦ Do′=Di′+t/2, Φ do′=di′−t/2    (t: clearance between the external teeth of the inner rotor ri and the internal teeth of the outer rotor ro), not only a clearance t/2 (tip clearance tt) is formed at the tip section as shown in FIG. 14 and FIG. 15, but a clearance (side clearance ts) between the tooth surfaces is also formed.
FIG. 13 to FIG. 15 show an oil pump rotor of an first example of conventional arts that meets the aforementioned conditions. As for the inner rotor ri of this oil pump rotor, the base circle bi′ has a diameter of Φ bi′=44.80 mm; the first outer rolling circle Di′ has a diameter of Φ Di′=3.60 mm; the first inner rolling circle di′ has a diameter of Φ di′=2.80 mm; and the teeth number is n=7. As for the outer rotor ro, the outer diameter thereof is Φ 65 mm; the base circle bo′ has a diameter of Φ bo′=51.20 mm; the second outer rolling circle Do′ has a diameter of Φ Do′=3.663 mm; the second inner rolling circle do′ has a diameter of Φ do′=2.737 mm; and the teeth number is (n+1)=8. In addition, the eccentricity amount is e′=3.2 mm.
As for the oil pump rotor of Japanese Patent No. 3734617 (referred to as first example of conventional arts hereunder) that has the aforementioned structure, the two rotors are so configured that the tooth shapes of the tooth tips of the inner rotor are formed smaller than the tooth shapes of the tooth grooves of the outer rotor, and that the tooth shapes of the tooth grooves of the inner rotor are formed larger than the tooth shapes of the tooth tips of the outer rotor. For this reason, a backlash and the tip clearance tt can respectively be set to be appropriately large, thereby making it possible to secure a large backlash while maintaining a small tip clearance tt. Thus, in a state where an oil pressure supplied to the oil pump rotor and a torque for driving the oil pump rotor are stable, it is possible to restrict the occurrence of the noises resulting from the collision between the external teeth of the inner side and the internal teeth of the outer side.
However, by adjusting the diameters of the second outer rolling circle Do′ and the second inner rolling circle do′ of the outer rotor in this manner, securing the tip clearance tt=t/2 shall inevitably cause the side clearance is to become large as shown in FIG. 14 and FIG. 15. Accordingly, the following problem remains unsolved with regard to the quietness of this oil pump rotor. That is, when the oil pressure occurring in the oil pump rotor is minute and the torque for driving the oil pump rotor changes, the internal teeth of the outer side and the external teeth of the inner side collide with one another such that collision energies at that time are turned into sounds. Those sounds can then be turned into noises after reaching an audible level.
An oil pump rotor configured in view of the aforementioned problem (e.g. Japanese Patent No. 4485770) has been proposed. As shown in FIG. 7 and FIG. 8, this oil pump rotor includes: an inner rotor 10 having “n” (n is a natural number) external teeth 11; an outer rotor 20 having “n+1” internal teeth 21 engageable with the external teeth 11; and a casing 50 having an intake port for a fluid to be drawn thereinto and a discharge port for the fluid to be discharged therefrom. Particularly, this oil pump rotor is used in an oil pump transferring a fluid by drawing in and discharging the same as volumes of cells formed between the tooth surfaces of the two rotors 10, 20 change when the two engaged rotors 10, 20 rotate. As for the aforementioned inner rotor 10, the shape of each tooth tip is established by an epicycloid curve that is generated by a first outer rolling circle Di externally tangent to and rolling on a base circle bi of the inner rotor 10 without slipping. The shape of each tooth groove of the inner rotor 10 is established by a hypocycloid curve that is generated by a first inner rolling circle di internally tangent to and rolling within the base circle bi without slipping. As for the aforementioned outer rotor 20, the shape of each tooth groove is established by an epicycloid curve that is generated by a second outer rolling circle Do externally tangent to and rolling on a base circle bo of the outer rotor 20 without slipping. The shape of each tooth tip of the outer rotor 20 is established by a hypocycloid curve that is generated by a second inner rolling circle do internally tangent to and rolling within the base circle bo without slipping. The inner rotor 10 and the outer rotor 20 are so configured that when the diameter of the base circle bi of the inner rotor 10 is Φ bi; the diameter of the first outer rolling circle Di is Φ Di; the diameter of the first inner rolling circle di is Φ di; the diameter of the base circle bo of the outer rotor 20 is Φ bo; the diameter of the second outer rolling circle Do is Φ Do; the diameter of the second inner rolling circle do is Φ do; and an eccentricity amount between the inner rotor 10 and the outer rotor 20 is e, the expression Φ bi=n·(Φ Di+Φ di) and the expression Φ bo=(n+1)·(Φ Do+Φ do) hold; the expression Φ Di+Φ di=2 e or Φ Do+Φ do=2 e holds; and the expressions Φ Do>Φ Di, Φ di>Φ do and (Φ Di+Φ di)<(Φ Do+Φ do) hold. Here, a backlash at an engagement point where a tooth tip of the outer rotor 20 and a tooth groove of the inner rotor 10 directly face each other; and a backlash during the process where the volumes of the cells increase and decrease, are smaller than a backlash at where the volume of a cell reaches its maximum level.
As for the oil pump rotor of Japanese Patent No. 4485770, the two rotors 10 and 20 exhibit small backlashes such that an oil pump rotor superior in quietness can be obtained. Particularly, the oil pressure occurring in the oil pump rotor is minute; and even if the torque for driving this oil pump rotor changes, noise occurrence due to the collisions between the internal teeth 21 of the outer side and the external teeth 11 of the inner side can be reliably restricted.